We investigate the properties and evolution of accretion tori formed after the coalescence of two compact objects. At these extreme densities and temperatures, the accreting torus is cooled mainly by neutrino emission produced primarily by electron and positron capture on nucleons (β-reactions). We solve for the disk structure and its time evolution by introducing a detailed treatment of the equation of state which includes photodisintegration of helium, the condition of β-equilibrium, and neutrino opacities. We self-consistently calculate the chemical equilibrium in the gas consisting of helium, free protons, neutrons, and electron-positron pairs and compute the chemical potentials of the species, as well as the electron fraction throughout the disk. We find that, for sufficiently large accretion rates (M˙>~10 Msolar s-1), the inner regions of the disk become opaque and develop a viscous and thermal instability. The identification of this instability might be relevant for GRB observations.